Expand.
Solution: $\begin{aligned} &\phantom{=}({8-3a^2})(2a^2+6) \\\\ &={8}(2a^2+6){-3a^2}(2a^2+6) \end{aligned}$ $\begin{aligned} &=16a^2+48-6a^4-18a^2 \\\\ &=-6a^4-2a^2+48 \end{aligned}$